James M Sim, forest officer, was the son of John Sim, a farmer and botanist, and a younger brother of the forest botanist Thomas Robertson Sim*. He was educated at the Public School, Banchory (just inland from Aberdeen), and at Aberdeen, and was occupied in nursery work. He made a special study of agricultural science, qualifying with first class honours at South Kensington, London, in agriculture and the related subjects agricultural botany and entomology. He also obtained the diploma of the Highland and Agricultural Society of Scotland in 1898 and was admitted as a Fellow of the society (FHAS). He practised horticulture as a hobby.
Sim came to the Cape Colony in 1899 and was employed in the Department of Forestry in April that year. From January 1901 he was stationed at Fort Cunynghame and in September 1902 became District Forest Officer at King William's Town. From July 1906 he was one of the District Forest Officers of the Eastern Conservancy, a position he still held in 1914. In 1906 he was seconded to government service in Southern Rhodesia (now Zimbabwe) for some time and compiled a report on forestry in that country. Extracts from his report were published as Bulletin No. 71 of the Rhodesia Department of Agriculture (Salisbury, 1911, 44p). In 1912 he wrote a report on forestry in Great Britain. In 1916 he published a paper on 'The modification of South African rainfall' (South African Journal of Science, 1916, Vol. 13, pp. 318-326) in which he argued that veld fires and forest destruction cause the soil to dry out and so reduce the transfer of moisture to the atmosphere. That same year he wrote an article on 'Erosion of the soil' in the Cape Times (14 January 1916).
Sim collected some mosses in the Eastern Cape for his brother. The specimens, as part of T.R. Sim's herbarium, went to the National Herbarium in Pretoria. He also collected fungi, and both he and his brother sent many fungi from Natal to I.B. Pole Evans* in Pretoria, where their specimens were incorporated in what is now the National Collection of Fungi of the Plant Protection Research Institute. For some time around 1910 he was a member of the South African Association for the Advancement of Science.
On 10 January 1901 Sim married Jessie Simpson of Kincardineshire, Scotland, with whom he had a son, J.T.R. Sim, who later became Professor of Agriculture at the University of Stellenbosch.